Attenuation Flashcards
What is attenuation?
The weakening of the sound beam as it travels
Is attenuation good or bad? Why is it important?
It is both pos and neg.
1) Limits our imaging depth (neg)
2) Must be compensated for (neg)
3) Can be useful in diagnostics (pos)
What is used to compensate for attenuation?
TGC’s and 2D Gain
What does TGC’s stand for?
Time Gain Adding Compensation
How is attenuation useful in diagnosis?
An increase in attenuation (shadow) behind a mass indicates that the mass is solid or stone. A cystic mass (fluid filled) would show a lack of attenuation (acoustic enhancement/through transmission/brightness) behind the structure.
What are the relative units of attenuation?
bel (B) and the decibel (dB)
What do bels and decibels express?
The loudness (intensity level) of soundwaves
1 bel = ?
10 decibels
How is the power of the sound leaving the transducer determined?
Instrument output using decibels
What is dynamic range?
A button on the machine controlling the amount of contrast.
More shades of grey & less contrast vs. Fewer shades of grey and more contrast.
How does dynamic range work?
Dynamic range uses decibels to express the number of shades of grey displayed on the monitor.
How do gain and TGC’s work?
They use decibels to express the amount of amplification required to optimize the returning echos.
What is the formula to calculate bels?
Bel = log(New I/original I)
What is the formula to calculate decibels?
dB = 10log(new I/original I)
What is the formula to calculate power?
dB = 10 log(new p/original p)
What is the formula to calculate voltage?
dB = 20log(new v/original v)
What is the trick for remembering the formulas for calculating intensity(bels + decibels), power and voltage?
Same formula but:
Bel = log
Decibel = 10log
Power = 10log
Voltage = 20log
What is the “rule of thumb” when dealing with decibels?
A 3dB drop = 1/2 the original intensity
A 10 dB drop = 0.1 of the original intensity
In a question calculating attenuation, is the “output” value the ‘new’ or ‘original’ value?
Output = New
Input = Original
Practice Intensity/power/voltage calculations using logs
.
What is an ‘attenuation coefficient’?
The amount of attenuation that occurs with one centimeter travelled.
What is the amount of attenuation in soft tissue for every centimeter travelled per 1 MHz?
0.5 dB per 1 MHz
What is the attenuation coefficient in soft tissue?
1/2 frequency
What is the formula for total attenuation?
TA = Attenuation Coefficient x path length (cm)